Vent openings for enclosed beverage containers to equalize internal pressures with outer or atmospheric pressures are well known. However, they have certain disadvantages. The vent opening must be small to limit possible leakage, and hence it often becomes plugged and rendered inoperative. Also, where the enclosed container is filled with very hot air or steam generated by hot tea or coffee, for example, the person carrying the container or pouring liquid therefrom might well be burned by the escaping gas or stream. Protective caps covering the exterior of the vent opening have been used, with limited success, as hot steam escaping from under the cap can still cause injury.
One such protective cap arrangement is disclosed in U.S. Pat. No. 3,064,668, issued Nov. 20, 1962. The cap has a stem extending through an outer wall of the container, with an axial bore extending through the stem and into a chamber in which a ball valve normally covers a vent hole in the bottom wall of the chamber and aligned with the bore. The upper end of the bore is in the outer cap and is connected with downwardly inclined exhaust channels opening out under the rim of the cap.
This prior venting system has certain disadvantages. Equalization of internal and external pressures is accomplished only when the container is tilted to roll the ball off the lower hole. Otherwise, when the internal pressure is lower than the outer atmosphere, inward flow is shut off by the ball over the lower hole. If the internal pressure increases, it forces the ball against the lower end of the bore, shutting off the outer atmosphere. Hence, internal pressure can build up in the ball chamber so that when tilted, the ball may not roll to open the bore or if opened, very hot blasts of steam may be exhausted from the cap.